Family groups suppress juveniles from becoming sexually active.

Older pups help raise their younger siblings.

Its a pretty well established fact that female pups help raise their parents next litter, this makes them an alloparent. Young females that stay with their parents reach sexual maturity at a slower rate. The presence of the father, mother, siblings and especially younger pups suppress the reproduction development of both female and male juvenile gerbils.
Many breeders choose to allow their breeding pairs to have two or three litters. There is some concern that female pups from the first litter might be impregnated once they are weaned. This is not the case. For the first ten weeks, it is unlikely that a female pup will go into heat unless she has been exposed to an unrelated and unfamiliar adult male. After ten weeks she may go into heat especially if the parents have stopped breeding themselves.
If you're interested, there is an study on how siblings affect the reproductive development of gerbils.